Electron microscopy catalyst

Occasionally, especially in the developmental phase of catalyst research, it is necessary to determine the oxidation state, exact location, and dispersion of various elements in the catalyst. Eor these studies, either transmission electron microscopy (TEM) or scanning electron microscopy (SEM) combined with various high vacuum x-ray, electron, and ion spectroscopies are used routinely. 

Oxide-supported metals constitute one of the most important classes of heterogeneous catalysts, and for this reason they have been investigated by many techniques adsorption isotherms, IR of chemisorbed molecules, electron microscopy, EXAFS, etc. Flowever, the fact that they have been studied by so many methods proves that no one technique is totally satisfactory. 

Regarding a historical perspective on carbon nanotubes, very small diameter (less than 10 nm) carbon filaments were observed in the 1970 s through synthesis of vapor grown carbon fibers prepared by the decomposition of benzene at 1100°C in the presence of Fe catalyst particles of 10 nm diameter [11, 12]. However, no detailed systematic studies of such very thin filaments were reported in these early years, and it was not until lijima s observation of carbon nanotubes by high resolution transmission electron microscopy (HRTEM) that the carbon nanotube field was seriously launched. A direct stimulus to the systematic study of carbon filaments of very small diameters came from the discovery of fullerenes by Kroto, Smalley, and coworkers [1], The realization that the terminations of the carbon nanotubes were fullerene-like caps or hemispheres explained why the smallest diameter carbon nanotube observed would be the same as the diameter of the Ceo molecule, though theoretical predictions suggest that nanotubes arc more stable than fullerenes of the same radius [13]. The lijima observation heralded the entry of many scientists into the field of carbon nanotubes, stimulated especially by the un-... 

Reactive compatibilization of engineering thermoplastic PET with PP through functionalization has been reported by Xanthos et al. [57]. Acrylic acid modified PP was used for compatibilization. Additives such as magnesium acetate and p-toluene sulfonic acid were evaluated as the catalyst for the potential interchange or esterification reaction that could occur in the melt. The blend characterization through scanning electron microscopy, IR spectroscopy, differential scanning calorimetry, and... 

The activity loss measured here is caused by recrystallizations. This was demonstrated by using scanning electron microscopy to determine nickel crystallite size in the same catalyst samples. These tests revealed that the catalyst used in demonstration plants has only a slight tendency to recrystallize or sinter after steam formation and loss of starting activity. 

MgO-supported model Mo—Pd catalysts have been prepared from the bimetallic cluster [Mo2Pd2 /z3-CO)2(/r-CO)4(PPh3)2() -C2H )2 (Fig. 70) and monometallic precursors. Each supported sample was treated in H2 at various temperatures to form metallic palladium, and characterized by chemisorption of H2, CO, and O2, transmission electron microscopy, TPD of adsorbed CO, and EXAFS. The data showed that the presence of molybdenum in the bimetallic precursor helped to maintain the palladium in a highly dispersed form. In contrast, the sample prepared from the monometallie precursors was characterized by larger palladium particles and by weaker Mo—Pd interactions. ... 

Determined by inductively coupled plasma-mass spectrometry of acid digested catalyst samples Calculated from X-ray diffraction peak broadening at (101) foranatase and (110) formtile TiOa Mean particle diameter measured from transmission electron microscopy pictures of gold catalysts... 

Because XPS is a surface sensitive technique, it recognizes how well particles are dispersed over a support. Figure 4.9 schematically shows two catalysts with the same quantity of supported particles but with different dispersions. When the particles are small, almost all atoms are at the surface, and the support is largely covered. In this case, XPS measures a high intensity Ip from the particles, but a relatively low intensity Is for the support. Consequently, the ratio Ip/Is is high. For poorly dispersed particles, Ip/Is is low. Thus, the XPS intensity ratio Ip/Is reflects the dispersion of a catalyst on the support. Several models have been reported that derive particle dispersions from XPS intensity ratios, frequently with success. Hence, XPS offers an alternative determination of dispersion for catalysts that are not accessible to investigation by the usual techniques used for particle size determination, such as electron microscopy and hydrogen chemisorption. 

In conclusion, XPS is among the most frequently used techniques in characterizing catalysts. It readily provides the composition of the surface region and also reveals information on both the oxidation state of metals and the electronegativity of any ligands. XPS can also provide insight into the dispersion of particles over supports, vrhich is particularly useful if the more common techniques employed for this purpose, such as electron microscopy or hydrogen chemisorption, can not discriminate between support and active phase. 

Thus, the interaction of the primary beam with the sample provides a wealth of information on morphology, crystallography and chemical composition. Using transmission electron microscopy to make a projection of the sample density is a routine way to study particle sizes in catalysts. 

Transmission electron microscopy is one of the techniques most often used for the characterization of catalysts. In general, detection of supported particles is possible, provided that there is sufficient contrast between particles and support - a limitation that may impede applications of TEM on well-dispersed supported oxides. The determination of particle sizes or of distributions therein is now a routine matter, although it rests on the assumption that the size of the imaged particle is truly proportional to the size of the actual particle and that the detection probability is the same for all particles, independent of their dimensions. 

Suppose you prepared an iron oxide catalyst supported on an alumina support. Your aim is to use the catalyst in the metallic form, but you want to keep the iron particles as small as possible, with a degree of reduction of at least 50%. Hence, you need to know the particle size of the iron oxide in the unreduced catalyst, as well as the size of the iron particles and their degree of reduction in the metallic state. Refer to Chapters 4 and 5 to devise a strategy to obtain this information. (Unfortunately for you, it appears that electron microscopy and X-ray diffraction do not provide useful data on the unreduced catalyst.)... 

The performance of a supported metal or metal sulfide catalyst depends on the details of its preparation and pretreatraent. For petroleum refining applications, these catalysts are activated by reduction and/or sulfidation of an oxide precursor. The amount of the catalytic component converted to the active ase cind the dispersion of the active component are important factors in determining the catalytic performance of these materials. This investigation examines the process of reduction and sulfidation on unsupported 00 04 and silica-supported CO3O4 catalysts with different C03O4 dispersions. The C03O4 particle sizes were determined with electron microscopy. X-ray diffraction (XRD), emd... 

High resolution electron microscopy has recently demonstrated the capability to directly resolve the atomic structure of surfaces on small particles and thin films. In this paper we briefly review experimental observations for gold (110) and (111) surfacest and analyse how these results when combined with theoretical and experimental morphological studies, influence the interpretation of geometrical catalytic effects and the transfer of bulk surface experimental data to heterogeneous catalysts. 

Analytical electron microscopy permits structural and chemical analyses of catalyst areas nearly 1000 times smaller than those studied by conventional bulk analysis techniques. Quantitative x-ray analyses of bismuth molybdates are shown from lOnm diameter regions to better than 5% relative accuracy for the elements 61 and Mo. Digital x-ray images show qualitative 2-dimensional distributions of elements with a lateral spatial resolution of lOnm in supported Pd catalysts and ZSM-5 zeolites. Fine structure in CuLj 2 edges from electron energy loss spectroscopy indicate d>ether the copper is in the form of Cu metal or Cu oxide. These techniques should prove to be of great utility for the analysis of active phases, promoters, and poisons. 

Analytical electron microscopy (AEM) permits elemental and structural data to be obtained from volumes of catalyst material vastly smaller in size than the pellet or fluidized particle typically used in industrial processes. Figure 1 shows three levels of analysis for catalyst materials. Composite catalyst vehicles in the 0.1 to lOim size range can be chemically analyzed in bulk by techniques such as electron microprobe, XRD, AA, NMR,... 

Analysis of individual catalyst particles less than IMm in size requires an analytical tool that focuses electrons to a small probe on the specimen. Analytical electron microscopy is usually performed with either a dedicated scanning transmission electron microscope (STEM) or a conventional transmission electron microscope (TEM) with a STEM attachment. These instruments produce 1 to 50nm diameter electron probes that can be scanned across a thin specimen to form an image or stopped on an image feature to perform an analysis. In most cases, an electron beam current of about 1 nanoampere is required to produce an analytical signal in a reasonable time. 

Figure 1. Three levels of analysis for catalyst materials, a) bulk analysis of an entire catalyst pellet, b) surface analysis and depth profiling from the surface inward, c) analytical electron microscopy of individual catalyst particles too small for analysis by other techniques.

Analytical electron microscopy (AEM) can use several signals from the specimen to analyze volumes of catalyst material about a thousand times smaller than conventional techniques. X-ray emission spectroscopy (XES) is the most quantitative mode of chemical analyse in the AEM and is now also useful as a high resolution elemental mapping technique. Electron energy loss spectroscopy (EELS) vftiile not as well developed for quantitative analysis gives additional chemical information in the fine structure of the elemental absorption edges. EELS avoids the problem of spurious x-rays generated from areas of the spectrum remote from the analysis area. 

Careful X-ray studies can indicate the validity of such a number, and, as well, reveal whether the catalyst particles cluster. In this case, the X-ray size will be much smaller than that indicated by chemisorption. This ctui also be done by comparing X-ray and electron microscopy results.(6)... 

However, in more recent times science has made rapid strides in this direction. It is now possible to use EXAFS in situ during a catalytic reaction to examine the average coordination of metal atoms in the small particles which often exist in precious metal catalysts [2]. High resolution transmission electron microscopy has evolved to the level of atomic resolution, but can only be used ex-situ, or in situ with moderate pressures when special cells are fitted [3]. 

The four doped tungsten oxide catalysts (noted above) were synthesized and used in this study. The catalysts were analyzed by scanning electron microscopy (SEM), x-ray difBaction (XRD), and electron... 

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